Field devices such as sensors and actuators are arranged within a plant, factory or other facility such a petrochemical plant, oil refining or pharmaceutical plant, where in the case of sensors they detect physical quantities, such as temperature, pressure and flow rate, and in the case of actuators transmit the control variable signals to controlling equipment such as pneumatic control valves or motor operated valves. Usually the field devices (e.g., sensors and actuators) are installed at a distant location to the control equipment. The control equipment is usually installed near to the central control room of the plants.
The sensor signals (or process variables) are transmitted as analog communication signals (such as 4 to 20 mA, 0 to 5V, 1 to 5V, etc.) over a pair of connected wires. The 4 mA to 20 mA current loop output is a standard interface for many industrial process control and factory automation systems. Similarly, the same method is used to send a control signal to process equipment such as control valves. The sensor and control valve along with a process controller forms a control (or current) loop. It is a general practice to install multiple devices (such as field displays, recorders etc.) in the same current loop in series. The analog wired communications from the field devices to the control room is passed through multiple junction boxes based on the wiring philosophy and location of the devices.
Although the process data is transmitted to the host system through an analog current signal, the current signal cannot be measured without breaking the current loop because a current measuring instrument (e.g., sensing resistor) needs to be connected in series with the loop to sense the current. However connecting a measuring device in series in the current loop forces the existing communications to be disconnected, albeit for a brief period of time. The general practice of repair to the wire continuity occurs at junction boxes. The junction boxes are typically prewired with terminal blocks and hence need to be opened to check the level of the current signal for the identified pair of wires.
Moreover, the process variable value is available only to the particular host system to which it is physically wired and connected. The process variable data can also be made available as a wireless signal using existing suitable adapters, which are limited in range (e.g., wireless HART and ISA100). However, it is expensive to create a wireless transmission capability at each process variable level without having dedicated wireless infrastructure in suitable locations and ranges. Conventional solutions in industrial control have many data hops to cross (level 0 to level 1 to level 2 to level 3 to level 4) for making the sensor data available on Internet (to allow connection to wireless devices) or to the level 4 network (e.g., operators in a control room).